Support for journald logging

[peterhoeg]

Is your feature request related to a problem? Please describe.

The default logger logs superflous info when logging to journald (of systemd fame):

Sep 06 12:04:53 day go-neb[2212178]: time="2021-09-06T12:04:53+08:00" level=info msg="Inserted 1 clients"

Describe the solution you'd like

journald handles both the level and timestamp, so all we really need is:

Sep 06 12:04:53 day go-neb[2212178]: msg="Inserted 1 clients"

It's possible to detect if you're running under systemd (and thus logging to journald) by checking for the environment variable INVOCATION_ID.

Describe alternatives you've considered

An alternative is to do proper logging of structured data to journald when
detected which would be nice (and which would also solve all of this), but just
getting rid of the duplicate information would be an excellent first step.